Fluorotyrosine M13 coat protein: fluorine-19 nuclear magnetic resonance study of the motional properties of an integral membrane protein in phospholipid vesicles

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Abstract
A fluorotyrosyl derivative of [phage] M13 coat protein was prepared in vivo and it was incorporated at high levels in small phospholipid vesicles, using a urea-cholate dialysis procedure. 19F NMR experiments at 254 MHz with this system indicate a [spin lattice relaxation time] T1 of 0.32 s and line width of 300 Hz. The observed line width increases dramatically below the gel to liquid-crystalline transition temperature for the lipid, indicating that the probe is sensitive to the phase state of the bilayer. Nuclear Overhauser enhancement and field dependence of line width were used to establish the relative contributions of dipolar interactions and chemical-shift anisotropy to the observed T1 and line width. From this relaxation data, a model for the motional properties of the protein in the lipid bilayer was constructed. This model is characterized by correlation times for rotation about the .alpha..beta. and .beta..gamma. bonds of the 2 tyrosyl residues of 2 .times. 10-8 and 5 .times. 10-9 s, respectively. Rapid intermolecular dipole-dipole interactions are required to account for the estimated dipolar contribution to T1. A reasonable model for these interactions is that lipid methylene protons are involved in relaxation of the fluorine probes (which reside in the hydrophobic region of this integral membrane protein). A minimum translational diffusion coefficient for such lipids of D .gtoreq. 3 .times. 10-9 cm2/s. was estimated.